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File Handling : Urgent

Posted on 2002-07-19
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Last Modified: 2010-05-19
Hi

I am writing one batch program in java
when it is exceuted it will pick up one CSV file to read.
wht i want is , Before it starts reading the file , i should rename the file so that other instance of the same batch program should not get the same file to read. After reading the file i want to rename the file back to its orignal name.

Thanks and Regards
Harish
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Question by:harish_tyagi
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CEHJ earned 50 total points
Comment Utility
Here's an adaption of the durable FileCopy class by David Flanagan. You can just call rename() in your program, giving it a prefix, such as rename("myfile.csv","_")

import java.io.*;

/**
 * This class is a standalone program to copy a file, and also defines a
 * static copy() method that other programs can use to copy files.
 **/
public class FileCopy {
  /** The main() method of the standalone program.  Calls copy(). */
  public static void main(String[] args) {
    if (args.length != 2)    // Check arguments
      System.err.println("Usage: java FileCopy <source file> <prefix>");
    else {
    // Call rename() to do the copy, and display any error messages it throws.
      rename(args[0],args[1]);
    }
  }

  public static void rename(String filename,String prefix){
     // Call copy() to do the copy, and display any error messages it throws.
    try {
      String newName = prefix + filename ;
      copy(filename,newName);
    }
    catch (Exception e) {
      System.err.println(e.getMessage());
    }
  }

  /**
   * The static method that actually performs the file copy.
   * Before copying the file, however, it performs a lot of tests to make
   * sure everything is as it should be.
   */
  public static void copy(String from_name, String to_name) throws IOException{
    File from_file = new File(from_name);  // Get File objects from Strings
    File to_file = new File(to_name);

    // First make sure the source file exists, is a file, and is readable.
    if (!from_file.exists())
      abort("FileCopy: no such source file: " + from_name);
    if (!from_file.isFile())
      abort("FileCopy: can't copy directory: " + from_name);
    if (!from_file.canRead())
      abort("FileCopy: source file is unreadable: " + from_name);

    // If the destination is a directory, use the source file name
    // as the destination file name
    if (to_file.isDirectory())
      to_file = new File(to_file, from_file.getName());

    // If the destination exists, make sure it is a writeable file
    // and ask before overwriting it.  If the destination doesn't
    // exist, make sure the directory exists and is writeable.
    if (to_file.exists()) {
      if (!to_file.canWrite())
        abort("FileCopy: destination file is unwriteable: " + to_name);
      // Ask whether to overwrite it
      System.out.print("Overwrite existing file " + to_name + "? (Y/N): ");
      System.out.flush();
      // Get the user's response.
      BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
      String response = in.readLine();
      // Check the response.  If not a Yes, abort the copy.
      if (!response.equals("Y") && !response.equals("y"))
        abort("FileCopy: existing file was not overwritten.");
    }
    else {
      // if file doesn't exist, check if directory exists and is writeable.
      // If getParent() returns null, then the directory is the current dir.
      // so look up the user.dir system property to find out what that is.
      String parent = to_file.getParent();  // Get the destination directory
      if (parent == null) parent = System.getProperty("user.dir"); // or CWD
      File dir = new File(parent);          // Convert it to a file.
      if (!dir.exists())
        abort("FileCopy: destination directory doesn't exist: " + parent);
      if (dir.isFile())
        abort("FileCopy: destination is not a directory: " + parent);
      if (!dir.canWrite())
        abort("FileCopy: destination directory is unwriteable: " + parent);
    }

    // If we've gotten this far, then everything is okay.
    // So we copy the file, a buffer of bytes at a time.
    FileInputStream from = null;  // Stream to read from source
    FileOutputStream to = null;   // Stream to write to destination
    try {
      from = new FileInputStream(from_file);  // Create input stream
      to = new FileOutputStream(to_file);     // Create output stream
      byte[] buffer = new byte[4096];         // A buffer to hold file contents
      int bytes_read;                         // How many bytes in buffer
      // Read a chunk of bytes into the buffer, then write them out,
      // looping until we reach the end of the file (when read() returns -1).
      // Note the combination of assignment and comparison in this while
      // loop.  This is a common I/O programming idiom.
      while((bytes_read = from.read(buffer)) != -1) // Read bytes until EOF
        to.write(buffer, 0, bytes_read);            //   write bytes
    }
    // Always close the streams, even if exceptions were thrown
    finally {
      if (from != null) try { from.close(); } catch (IOException e) { ; }
      if (to != null) try { to.close(); } catch (IOException e) { ; }
    }
  }

  /** A convenience method to throw an exception */
  private static void abort(String msg) throws IOException {
    throw new IOException(msg);
  }
}
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Expert Comment

by:CEHJ
Comment Utility
Actually, I think it might be better to keep the original version of Flanagan's FileCopy, so here's a test class to show how you'd call it, followed by the original:

import java.io.*;

public class TestFileCopy {

  public static void main(String[] args){
    try {
      String fileName = "myfile.csv";
      String prefix = "_";
      String newName = prefix + fileName;
      FileCopy.copy(fileName,newName);
      /*
      do your stuff
      */
      File f = new File(newName);
      System.out.println(f.length());
      f.delete();
    }
    catch(IOException e){
      e.printStackTrace();
    }
  }

}


----------------------------------------------------------

// This example is from _Java Examples in a Nutshell_. (http://www.oreilly.com)
// Copyright (c) 1997 by David Flanagan
// This example is provided WITHOUT ANY WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for non-commercial purposes.
// For any commercial use, see http://www.davidflanagan.com/javaexamples

import java.io.*;

/**
 * This class is a standalone program to copy a file, and also defines a
 * static copy() method that other programs can use to copy files.
 **/
public class FileCopy {
  /** The main() method of the standalone program.  Calls copy(). */
  public static void main(String[] args) {
    if (args.length != 2)    // Check arguments
      System.err.println("Usage: java FileCopy <source file> <destination>");
    else {
      // Call copy() to do the copy, and display any error messages it throws.
      try { copy(args[0], args[1]); }
      catch (IOException e) { System.err.println(e.getMessage()); }
    }
  }

  /**
   * The static method that actually performs the file copy.
   * Before copying the file, however, it performs a lot of tests to make
   * sure everything is as it should be.
   */
  public static void copy(String from_name, String to_name) throws IOException{
    File from_file = new File(from_name);  // Get File objects from Strings
    File to_file = new File(to_name);
   
    // First make sure the source file exists, is a file, and is readable.
    if (!from_file.exists())
      abort("FileCopy: no such source file: " + from_name);
    if (!from_file.isFile())
      abort("FileCopy: can't copy directory: " + from_name);
    if (!from_file.canRead())
      abort("FileCopy: source file is unreadable: " + from_name);
   
    // If the destination is a directory, use the source file name
    // as the destination file name
    if (to_file.isDirectory())
      to_file = new File(to_file, from_file.getName());
   
    // If the destination exists, make sure it is a writeable file
    // and ask before overwriting it.  If the destination doesn't
    // exist, make sure the directory exists and is writeable.
    if (to_file.exists()) {
      if (!to_file.canWrite())
        abort("FileCopy: destination file is unwriteable: " + to_name);
      // Ask whether to overwrite it
      System.out.print("Overwrite existing file " + to_name + "? (Y/N): ");
      System.out.flush();
      // Get the user's response.
      BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
      String response = in.readLine();
      // Check the response.  If not a Yes, abort the copy.
      if (!response.equals("Y") && !response.equals("y"))
        abort("FileCopy: existing file was not overwritten.");
    }
    else {  
      // if file doesn't exist, check if directory exists and is writeable.
      // If getParent() returns null, then the directory is the current dir.
      // so look up the user.dir system property to find out what that is.
      String parent = to_file.getParent();  // Get the destination directory
      if (parent == null) parent = System.getProperty("user.dir"); // or CWD
      File dir = new File(parent);          // Convert it to a file.
      if (!dir.exists())
        abort("FileCopy: destination directory doesn't exist: " + parent);
      if (dir.isFile())
        abort("FileCopy: destination is not a directory: " + parent);
      if (!dir.canWrite())
        abort("FileCopy: destination directory is unwriteable: " + parent);
    }
   
    // If we've gotten this far, then everything is okay.
    // So we copy the file, a buffer of bytes at a time.
    FileInputStream from = null;  // Stream to read from source
    FileOutputStream to = null;   // Stream to write to destination
    try {
      from = new FileInputStream(from_file);  // Create input stream
      to = new FileOutputStream(to_file);     // Create output stream
      byte[] buffer = new byte[4096];         // A buffer to hold file contents
      int bytes_read;                         // How many bytes in buffer
      // Read a chunk of bytes into the buffer, then write them out,
      // looping until we reach the end of the file (when read() returns -1).
      // Note the combination of assignment and comparison in this while
      // loop.  This is a common I/O programming idiom.
      while((bytes_read = from.read(buffer)) != -1) // Read bytes until EOF
        to.write(buffer, 0, bytes_read);            //   write bytes
    }
    // Always close the streams, even if exceptions were thrown
    finally {
      if (from != null) try { from.close(); } catch (IOException e) { ; }
      if (to != null) try { to.close(); } catch (IOException e) { ; }
    }
  }

  /** A convenience method to throw an exception */
  private static void abort(String msg) throws IOException {
    throw new IOException(msg);
  }
}
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LVL 92

Assisted Solution

by:objects
objects earned 50 total points
Comment Utility
Sounds like you just need to rename the file:

File original = ...;
File newname = ...;
original.renameTo(newname);
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Expert Comment

by:girionis
Comment Utility
No comment has been added lately, so it's time to clean up this TA.

I will leave a recommendation in the Cleanup topic area that this question is:

- split points between CEHJ and objects

Please leave any comments here within the
next seven days.

PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER !

girionis
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